PROJECT SUMMARY Despite recent advances in treatment, metastatic castration resistant prostate cancer (mCRPC) remains incurable, and approximately 30,000 men die of this disease yearly. Advances in immunotherapy with drugs targeting immune checkpoints have raised hopes that these agents will improve outcomes for mCRPC patients. While initial studies of immune checkpoint blockade have been unsuccessful, emerging evidence suggests a subset of prostate cancer (PCa) patients can respond, although the mechanisms of PCa immunotherapy response and resistance are incompletely characterized. Work in other immunotherapy responsive malignancies has found several predictive immune and tumor-intrinsic properties that contribute to response, but the extent to which these (or other) features are operant in PCa is largely unknown. For example, we recently identified mutations in a chromatin remodeling complex that mediates immunotherapy response through T cell interactions in solid tumors, and in parallel discovered a previously unknown PCa genomic subclass defined by mutations in these same chromatin remodelers. These findings indicate that tumor-intrinsic epigenetic dysregulation may also interact with the immune system to modulate PCa immunotherapy responsiveness. The overarching hypothesis of this project is that multiple immune and tumor- intrinsic properties mediate PCa interactions with the immune system, and these interactions can be modified through selective targeting in combination with checkpoint blockade to expand the therapeutic potential of immunotherapy in PCa. We will leverage our team's deep experience in clinically grounded molecular characterization and preclinical models that can test immunotherapy combinations in PCa to define the processes that govern the immunotherapy landscape in PCa. The proposed specific aims are: 1) Define the systemic and infiltrating immune states in PCa associated with clinical response to checkpoint blockade; 2) Establish the immunologic impact of chromatin dysregulation and inhibition in PCa; and 3) Determine the impact of existing DNA damaging agents for sensitizing PCa to PD-1 blockade. This proposal leverages the extensive, novel, and complementary resources at both Dana-Farber/Broad Institute and University of California, San Francisco, led by highly collaborative investigators and an international scientific team, to address the hypotheses outlined herein. Through a combination of functional, molecular, and clinical approaches inherent in these studies, our team is poised to identify mCRPC cohorts that may benefit from this treatment paradigm, determine strategies to augment the use of checkpoint inhibitors in this disease, and mechanistically define the immune and tumor-intrinsic defects that drive immunoresistance in PCa. Broadly, this project will provide a unique approach for the Immuno-Oncology Translation Network (IOTN) community and enable discovery of anti-cancer immunotherapies strategies for PCa that may have larger relevance across the IOTN network and collaborating members of the Cancer Immunotherapy Consortium.